1. Field
The present Application relates generally to the operation of multimedia transmission systems, and more particularly, to methods and apparatus for providing fast service acquisition.
2. Background
In current content delivery/media distribution systems, when an application involves the transmission of a multiplex of compressed media (video and audio) where a receiver tunes to one of the channels in the multiplex, the latency in switching between channels is key to the user's experience. For example, in a conventional multimedia broadcast system, a multimedia server consists of a bank of source encoders that compress individual pieces of media that feed into a multiplexer, which consolidates the compressed media into a single multiplexed stream. The broadcast server communicates the compressed content to a broadcast receiver over a broadcast network, which could be heterogeneous in nature and error-prone. The broadcast receiver receives at least a portion of the multiplex and a de-multiplexer extracts the media of interest. Typically, there is a client application that enables “tuning” to the media channel/program of interest. This may or may not be through user intervention.
When the source is video, channel switching is possible only at random access points (RAPs) in the compressed/coded video bitstream. These RAPs comprise intra-frames (I-frames that are independently decodable) or through progressive I-frames (which are I-blocks that are distributed over more than one video frame). The channel switching time depends on the frequency of such random access points, which is typically between 2-10 seconds (since RAPs increase the average bit rate and hence transmission bandwidth of the coded video bitstream).
Random access through I-frames and distributed I-frames for progressive intra refreshes as proposed in industry standard H.264 are the most popular approaches thus far to service acquisition at the cost of quality and bandwidth. Switching latencies in these cases are on the order of a few seconds.
In some systems channel switching and trick play (step through, fast forward, rewind) are enabled through intra frames placed/coded periodically and as often as appropriate in video streams. However, I-frames increase the bit-rate and bandwidth of the video streams significantly. Hence the frequency of I-frames is typically from one second to ten seconds in typical applications. This implies that a channel switch can, at best, occur in one second (when all required conditions are met, for example, channel switching was enabled just before an I-frame and the buffer has just refreshed.
Progressive intra refresh enables channel switching in an incremental fashion. The prediction needs to be controlled such that a complete frame is refreshed in a pre-determined duration. In this case, channel switching includes latency equal to this duration in addition to latencies induced by buffer depths and switching at lower layers.
Thus, for example, if a device is receiving a multiplex that contains one hundred compressed media channels and the device user wishes to switch between channels, conventional system may take between 1-10 seconds to perform each channel switch. The time for each channel switch is generally dependant on when in the transmission frame the switch request occurs. Thus, device users experience long and varying delay times when switching between channels, which can be frustrating and result in an unsatisfactory user experience.
Therefore, what is needed is a system that operates to provide fast service acquisition and/or switching between services in a multiplex.